Hydrogen is used in many industrial fields, in particular as fuel or reagent (for example, for hydrogenation reactions). In this context, due to its volume in the gas state and its explosiveness in air, it is desirable for the hydrogen to be stored in a form that guarantees reduced size and safe containment.
The most common storage mode today consists in compressing the hydrogen gas. Said storage, called hyperbaric, is done at a pressure between 350 and 700 bar. Thus, the tanks employed must withstand high pressures and are therefore costly. Furthermore, the materials and the structure of these tanks are found to age poorly, raising problems of safety beyond a certain number of filling cycles.
Another storage mode consists in liquefying the hydrogen in cryogenic tanks at low temperature (−253° C.). One of the major drawbacks of this solution is the insulation of the tanks, especially in mass public applications. This is because despite efficient insulation, hydrogen contained in these tanks is reheated and is then converted to gas and escapes from the tank. This process, called boil-off, generates losses, precluding its application in closed premises.
The above two types of storage also require a large amount of energy to compress or cool the hydrogen. The energy balance of the use of hydrogen with these storage modes is therefore poor.
In recent years, hydrogen storage in the form of metal hydride has been investigated as an advantageous alternative, allowing for safer storage conditions and limited energy expenditure.
Some metals or alloys can reversibly incorporate hydrogen atoms in the crystal lattice. The hydrogen is absorbed/desorbed by these materials according to the temperature and pressure conditions. Examples include palladium (Pd), magnesium (Mg), ZrMn2, Mg2Ni, and alloys such as Mg—Mg2Ni and alanates.
As used here, depending on the step of the process, the term metal hydride also covers the metal partially or completely loaded with hydrogen.
A distinction is generally made between two types of metal hydride: heavy hydrides (mainly LaNi5, and alloys such as ferro-titanium alloy or Ti—V—Cr based alloy) and light hydrides (mainly magnesium and lithium).
With heavy hydrides, the hydrogen is absorbed at ambient temperature and pressure. The exothermicity of the reaction is generally moderate (not exceeding 35 kJ/mol H2). During use, the hydrogen is then desorbed at ambient temperature and pressure. The energy input required to use the hydrogen is reasonable. These heavy hydrides are therefore generally recommended for supplying hydrogen for fuel cells.
On the contrary, with light hydrides, the absorption of hydrogen by the light metal hydride requires a higher temperature (about 300° C. for MgH2). This reaction is highly exothermic (75 kJ/mol H2). The energy input required to initiate the hydrogen absorption reaction is therefore moderate. However, the absorption reaction is spontaneously interrupted if the heat generated is not removed. Furthermore, during use, the hydrogen desorption requires a high heat input, because the reaction is endothermic. The use of light hydrides therefore requires very accurate thermal management, during both the absorption and desorption of the hydrogen.
The absorption of one mole of H2 liberates 75 kJ/mol, whereas its subsequent combustion liberates only 250 kJ/mol, hence a thermal efficiency of about 70% if the heat of reaction is not recovered. It is also necessary to consider the efficiencies of internal combustion engines (about 27%) or fuel cells (about 60%), which shows that using this storage mode offers no advantage in energy terms, unless the heat energy (75 kJ/mol) is recovered.
The present invention proposes to recover the heat energy of absorption and to use it for desorption in order to obtain a satisfactory overall efficiency.
However, this appears to be dangerous and ineffective. Thus, it has already been proposed, in patent EP 0 015 106, to construct a tank for metal hydride powder, comprising a molten salt medium for storing the heat of the exothermic absorption reaction and releasing this heat during the endothermic desorption.
However, due to their very low thermal conductivity (about 0.5 W/m.K), the salt fusion kinetics is 3 to 10 times slower compared to the materials of the present invention. Patent EP 0 015 106 excludes any possibility of operation at high thermal power levels. Furthermore, molten salts, in addition to their low thermal conductivity, are corrosive and even, in some cases, toxic or explosive. In case of accidental leakage, the reaction between the molten salt and the metal hydride is extremely violent. These salts also have a great difference in density between their solid and liquid phases, which cause substantial shrinkage cavities.